Understanding the exact turbulent flow conditions that result in sediment entrainment has been a topic of investigation by scientists and researchers for decades (Pähtz et al., 2020). Of special interest is the determination of the critical conditions for the initiation of motion of sediment particles (incipient motion). Among all the criteria that exist in literature to determine the conditions for incipient motion, the impulse and energy criteria (e.g. Diplas et al., 2008; Valyrakis et al., 2010; Valyrakis et al., 2013) are considered to truly capture the dynamic nature of flow turbulence responsible for coarse particle entrainment. The impulse criterion states that the magnitude as well as the duration of the force of a turbulent flow structure are important for the removal of a particle from the riverbed surface. In this work, the sufficiently energetic turbulent flow structures that are able to entrain coarse particles resting on a bed surface when advected over them, are characterised. Well-controlled flume experiments are conducted for a range of flowrates at near-threshold conditions, as defined by the entrainment of a miniaturised instrumented particle (3cm). The particle is embedded with micro-electro-mechanical (MEMS) sensors that consist of accelerometer, gyroscope and magnetometer that enables it to track its motion and quantify its inertial dynamics (Al-Obaidi et al., 2020). Acoustic Doppler velocimetry (ADV) is used to take local flow hydrodynamic measurements. Quadrant analysis is used to characterise the strong flow events. It is found that majority of the quadrants for the sufficiently energetic turbulent flow structures that can lead to entraining coarse particles from a bed surface are sweeps (Q4). Additionally, the vast majority of the transition from one quadrant to another has existed is a transition from Q1 to Q4, outward interactions to sweep, for a short period (here 0.04s) before the occurrences of the strong turbulent flow events.
Keywords: impulse criterion; energy criterion; sediment entrainment; incipient motion; quadrant analysis; instrumented particle; MEMS.
References:
Al-Obaidi, K.; Xu, Y.; Valyrakis, M. The Design and Calibration of Instrumented Particles for Assessing Water Infrastructure Hazards. J. Sens. Actuator Netw. 2020, 9, 3, 1-18, https://doi.org/10.3390/jsan814323.
Diplas, P.; Dancey, C.L.; Celik, A.O.; Valyrakis, M.; Greer, K.; Akar, T. The Role of Impulse on the Initiation of Particle Movement Under Turbulent Flow Conditions. Science 2008, 322, 5902, 717-720, doi:10.1126/science.1158954.
Pähtz, T.; Clark, A. H.; Valyrakis, M.; Durán, O. The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments. Rev. Geophys. 2020, 58, 1, 1-58, doi:10.1029/2019RG000679.
Valyrakis, M.; Diplas, P.; Dancey, C.L.; Greer, K.; Celik, A.O. Role of instantaneous force magnitude and duration on particle entrainment. J. Geophys. Res. Earth Surf. 2010, 115, 1–18, doi:10.1029/2008JF001247.
Valyrakis, M.; Diplas, P.; Dancey, C.L. Entrainment of coarse particles in turbulent flows: An energy approach. J. Geophys. Res. Earth Surf. 2013, 118, 42–53, doi:10.1029/2012JF00235.
